You should think about where you get the email addresses
that you send email to. Never send email to addresses that you get from
outside sources. If you do use a list from someone else, and you start
to get spam reports, dump the whole list immediately. The fact that
a few users from a list are reporting spam indicates that many other
addresses on the same list are probably 'dirty.'
If you do get email addresses from
multiple sources, keep track of where each one comes from so that you can
figure out where the "dirty" addresses are coming from.

State your terms clearly

Make it very clear to people when they sign up for your list what they
should expect. If you send your email once a week, let them know that they
will be getting weekly email from you, and tell them what they will have to
do in the future to unsubscribe.

If you offer some service in return for people reading your email, make
sure that people know reading your email is a condition of a contractual agreement.

Verify email addresses

Sometimes, people will use a fake email address when signing up for something. Once in a while, this fake address can be someone else's real address. The recipient then sees your mailing as spam. Also, people have been known to take revenge on spam-fighters by signing them up for hundreds of mailing lists at once. Ideally, all mailing lists should protect against this. You can protect against this by asking each list subscriber to respond with a special code that you send out in email:

New user (Joe) asks to be added to list.

You send email with enable-code to Joe confirming signup.

Joe replies to your email.

Signup confirmed, and Joe is added to your list.

This is the method most noncommercial lists use, as well as well run commercial systems like deja.com.

This is a bit of a barrier for a commercial list, so you may decide to avoid verification of this kind, but you should be extra careful in your removal procedures if you do forgo address verification. You should also be prepared for fallout from this (IMHO) bad decision.

Purchased Lists

Purchased lists are bad. Period. The people on those lists didn't opt-in to receive email from you. They're not going to recognize you, and they're going to report your mail as spam. Besides possibly ending up on the SpamCop and other blocking lists, you could find yourself blocked from sending to many of the major providers such as AOL, Hotmail, Yahoo and Mail.com. Doing email right means getting permission. People should know they're signing up to receive emails from you. If they don't, that list will cause problems.

Identify yourself clearly

At the start of your mailing, identify your company and tell the recipients where you got their email addresses and when/where they authorized you to use their addresses for sending this mailing. For example "This email is sent from www.widgets.com. You authorized this mailing when you registered your widget on our web-site. See below for removal directions."

Personalize the headers and body of the email

It takes longer to send mailings this way, but it makes the mailing look much more credible and professional. Personalize the email with the recipient's email address in the To: field, and at the start of the body - possibly as part of the sender-identification suggested above. "Hello buyer@widgetbuyers.com. This email.."
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